421 email error is typically a temporary routing issue on the receiving mail server's end. These should usually be resolved automatically, and you should receive a final bounce-back message if the message completely fails.

You usually would not receive a bounce-back message for this type of error. If your message is going to be delayed more than 24 hours, then sometimes a "delivery has been delayed" soft bounce will be returned to you, to let you know that the delivery has yet to complete succesfully.

Common variations of the 421 email error

  • 421 4.2.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html
  • 421 4.3.1 Out of disk space for temp file
  • 421 4.3.2 All server ports are busy
  • 421 4.3.2 All server ports are busy
  • 421 4.4.2 mx.aol.com Error: timeout exceeded
  • 421 #4.4.5 Too many connections from your host.
  • 421 #4.4.5 Too many connections to this host.
  • 421 4.7.0 mx.aol.com Error: too many errors
  • 421 4.7.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html
  • 421 4.7.1 POL-T2 http://postmaster.facebook.com/response_codes?ip= Connection refused
  • 421 4.7.1 POL-T2 http://postmaster.facebook.com/response_codes?ip= Connection refused

  • 421 Downstream server error
  • 421 Fork failed
  • 421 Greylisting enabled, please try later.
  • 421 LDAP verification agent temporarily unavailable: Connection failed
  • 421 mail.RemoteDomain.com out of connection slots
  • 421 mx.aol.com Service unavailable - try again later
  • 421 <recipient@RemoteDomain.com> Service not available - too busy
  • 421 Server busy, try again later - psmtp
  • 421 service temporarily unavailable
  • 421 Temporary error (#4.3.0)
  • 421 Too many concurrent SMTP connections; please try again later.
  • 421 Unexpected failure, please try later

Resolving a 421 email error

Because errors in the 421 series don't bounceback right away they would be considered a soft bounce. To better understand soft and hard bounces, and general email bounceback errors you can read why does email bounce, bounceback, or error?

Seeing a large amount of 421 email errors in your server's mail logs could be an indication that a lot of your mail users are having difficulties getting their e-mails delivered out successfully on the first attempt.

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